Vito uses $9$ liters of water to water $24$ flower pots. He is wondering how many liters of water $(w)$ it would take to water $40$ flower pots. He assumes he'll use the same amount of water on each pot. How many liters of water does it take to water $40$ flower pots?
We can set up a proportion like this: $\dfrac{\text{Water needed for 40 pots}}{\text{Water needed for 24 pots}} = \dfrac{\text{40 pots}}{\text{24 pots}}$ Substituting values from the problem, we get this: $\dfrac{w \text{ liters}}{9\text{ liters}} = \dfrac{40\text{ pots}}{24\text{ pots}}$ Now, solve the proportion for $w$ : $\begin{aligned} \dfrac{w}{9} &= \dfrac{40}{24} \\\\ \dfrac{w}{9} &= \dfrac{5}{3} \\\\ w &= \dfrac{5}{3} \cdot 9 \\\\ w &= \dfrac{5\cdot 9}{3} \\\\ w &= \dfrac{5\cdot \stackrel{3}{\cancel9} }{\underset{1}{\cancel3}} \\\\ w &= \dfrac{5 \cdot 3}{1} \\\\ w &= \dfrac{15}{1} \\\\ w &= 15 \end{aligned}$ It takes $15$ liters of water to water $40$ flower pots.